[Developers] Refactor musical games to use a CVT between game and song

[Bryan Cheung]

https://bugs.freebase.com/browse/DA-753

The properties /cvg/musical_game/songs and /cvg/musical_game_song/ 
playable_in_game should be refactored to expect a new CVT: /cvg/ 
musical_game_song_relationship.

This will break queries that expect a simple link relationship to  
exist.  Is there anyone that has queries that this change will affect  
adversely?

Bryan